The invention relates to process control systems and, in particular, to a system for controlling a process yet permitting the process to drift from a fixed set of conditions.
In the manufacture or treatment of articles using chemical processes, the quality of the product is determined, in part, by how well various process parameters are controlled, e.g. temperature and pressure. As a specific example, in the manufacture of semiconductor devices, a silicon wafer is subjected to a long sequence of steps in which each step has a number of parameters associated with it. As a more specific example, one may choose to deposit a layer of metal upon an oxide layer previously formed on a wafer. There are several ways to do this, but if one chooses plasma enhanced Chemical Vapor Deposition (CVD) the list of parameters includes RF power, pressure within the chamber, flow of each gas used in the mixture, wafer temperature, timing of events within the reactor. Uncontrolled parameters, in the sense that they are not directly controlled, are deposition rate, uniformity, and resistivity. One usually tries to obtain maximum deposition rate consistent with good uniformity and low resistivity.
As well known in the art, the results obtained are a compromise among competing effects. For example, one can increase deposition rate, but at the expense of uniformity. On the controlled side of the process, the parameters are balanced to give the optimum result. This often entails rather tight restrictions on the controllable parameters. When one attempts to automate a process to reduce human error or system errors, the result is frequent shutdowns because the process has drifted out of specification. Typically, the operator knows what parameter was out of specification, but has little else to indicate the source of the problem or the cure. The downtime often makes the automatic equipment seem even less efficient than the equipment it replaced.
In view of the foregoing, it is therefore an object of the present invention to provide a control system tolerant of process drift.
Another object of the present invention is to provide a process control system which facilitates finding causes of shutdowns.
A further object of the present invention is to provide a process control system which enables dynamic adjustment of process parameters.